The broad purpose of the research in this proposal is to understand how microenvironments (secondary coordination spheres) about metal ions control function. A bio-inspired synthetic approach is utilized that incorporates principles of molecular architecture found in the active sites of metalloproteins into synthetic systems. Multidentate ligands will be developed that create rigid organic structures around metal ions and place hydrogen bond donors or acceptors proximal to the metal centers, forming specific microenvironments. One distinguishing attribute of these systems is the ability to make site-specific modifications to the structure in order to evaluate correlations between the microenvironment and reactivity. A focus of this research is the examination of transient intermediates that are formed from the activation of dioxygen and the oxidation of water - processes that are directly linked to the maintenance of human health and aging. Long-term goals include developing structure-function relationships in metal- assisted oxidative catalysis. Metalloproteins perform functions not yet achieved in synthetic systems. Our hypothesis is that the lack of control of the secondary coordination sphere in synthetic compounds is a major obstacle in establishing the desired functions. Results from structural biology show that hydrogen bonds within the secondary coordination spheres of metalloproteins are instrumental in regulating function. Therefore, the function and dysfunction of health-related metalloproteins can be understood in the context of changes in their microenvironments. However, it is still unclear, even in biomolecules, how non-covalent interactions influence metal-mediated processes. Investigations into these effects require fundamental reactivity and mechanistic studies in which the contributions of single components can be analyzed individually. We have developed synthetic hydrogen bonding systems in which the molecular components that define the structure around the metal ion are specifically controlled; in turn, this permits the formation of systems whose activity can be tailored to a particular function. This ability to regulate the microenvironment allows for systematic studies into structure-function relationships that lead to fundamental understanding of chemical processes. Ultimately, this research will provide insights into the properties of biological catalysts and lead to new classes of synthetic catalysts that exhibit the exquisite control over reactivity that is characteristic of metalloenzymes.